The present invention relates to the cooling of integrated circuit (IC) packaging structures. More particularly the present invention is directed to the cooling of multiple chips, or ICs, using a common heat spreader or lid structure where the chips have different cooling requirements.
As heat is generated during the functioning of integrated circuit chips (ICs), the thermal resistance to the heat sink must be small so that the operating temperature of the chip is low enough to assure the continued reliable operation of the device. The problem of heat removal becomes more difficult as chip geometries are scaled down and operating speeds are increased, resulting in increased power density. The ability to adequately cool the chips can therefore be a limiting factor in the further increase of system performance. Integrated circuit chips mounted face down on wiring substrates, and particularly multiple chips on a common substrate such as is found in a multichip module (MCM), present special cooling difficulties. In an MCM, the chips may be mounted very close together and nearly cover the whole top surface of the MCM wiring substrate.
Additionally, the chips may have different cooling requirements. For example, a processor chip may have a higher power density (W/cm2) than a memory chip mounted on the same wiring substrate, or first level package. Further, the maximum allowed device junction temperature may be different for different chips mounted to the same wiring substrate. An additional problem is that processor chips, and other chips, frequently have a “hot spot” which can have a heat flux (W/cm2) significantly greater than the average heat flux resulting in temperatures approximately 20° C. hotter than the average chip temperature. A thermal solution which may be adequate for the average chip power density may not be adequate to allow reliable operation of the hot spot region of the chip.
A common technique for reducing the thermal resistance from a high-power IC to a heat sink, or cooling plate, is to add an intermediate heat spreader between the chip and the heat sink, or cooling plate, to reduce the power density (W/cm2) which passes through the layer of compliant thermally conductive material. The addition of a heat spreader can be advantageous because the complaint thermally conductive material layer is usually a significant fraction of the total thermal resistance. Generally, due to the difference in thermal expansion between the chip and the material of the cooling plate or heat sink, which is typically copper or aluminum, a compliant thermally conductive material is required between the chip and the heat sink or cooling plate.
Silicon has a linear thermal coefficient of expansion (TCE) of about 2.5 ppm/° C., copper has a linear thermal coefficient of expansion of about 16.5 ppm/° C. and aluminum has a linear thermal coefficient of expansion of about 23 ppm/° C. The heat sink, or cooling plate, will conduct heat away from the IC chip(s) and the heat is removed from the cooling plate or heat sink by methods such as forced air cooling or circulating liquid coolants. Compliant thermally conductive materials are typically thermal pastes, or thermal greases and are frequently referred to as thermal interface materials, or TIMs. The total thermal resistance between the chip and heat sink or cooling plate can usually be reduced by rigidly bonding to the back surface of the chip a heat spreader. The heat spreader is composed of a material with a coefficient of thermal expansion close to that of Si, and with a high thermal conductivity.
Suitable materials for such heat spreaders include diamond (TCE of about 2.8 ppm/° C.), composites of Si and SiC, referred to as SiSiC, SiC (TCE of about 4 ppm/° C.), Mo, and various ceramic materials, where diamond or a SiSiC composite are most suitable. Suitable materials also include composite materials containing, Si or SiC or diamond or ceramic particles such as copper filled with SiC particles. Due to the close thermal expansion match, a rigid bond with a thermal adhesive or solder can be used and a compliant layer is not needed. A heat sink is than mounted onto the thermal spreader using a layer of a compliant thermally conductive material. Note that in some packaging structures, the lid, or package cap, can also serve as a heat spreader. With rigid thermal bonding methods, using thermal adhesives such as silver filled epoxy, or other filled polymers, or solders, a lower thermal resistance join can be formed than with TIMs.
With a thermal adhesive such as a silver filled epoxy or filled polymer, a typical bond thickness is about 12 microns, and may range from 6 to 12 microns and will be reduced to around 5 microns in the future. With solders, or metal layers, used for thermally joining a chip to a thermal spreader, a typical bond thickness is about 25 to 50 microns and will be reduced to around half that in the future. As mentioned above, multiple chips may be attached to a common wiring substrate (first level package) and have a common thermal spreader, or package cap. It is desirable to use a common thermal spreader, or package cap, to minimize the number of parts and simplify the package assembly. When multiple chips are attached face down to a common wiring substrate made from a ceramic material with micro solder balls (C4's), the typical chip to chip height variation is about +/−20 microns.
Edwards, U.S. Pat. No. 5,757,620, is directed at customizing the cooling of different chips on a MCM by varying the depth of thermal compound filled gaps, or blind holes that are above each chip. The cap is typically metallic or ceramic, a plurality of semiconductor elements such as chips are secured to a substrate or module via a plurality of solder connections, and customized thermal contact areas are formed and are different for their corresponding chips and the thermal paste filled customized thermal contact areas provide a heat or thermal transfer path between the corresponding chips and the heat sink.
Sherif et al., U.S. Pat. No. 5,623,394, is directed at customizing the cooling of different chips on a MCM by using a plurality of thermally conductive materials. The heat sink or cap has a plurality of extensions or partitions that form uniform cavities or blind holes. The thermally conductive paste or grease fills the uniform cavity and provides the heat or thermal transfer path from the chips to the heat sink or cap where the appropriate thermally conductive material is placed between the chip and the thermal cap. Thermal paste or compounds could be thermal materials having a higher or lower thermal conductivity, or could have any thermal conductivity in between. Both of the above references require complicated structures to be formed on the cap or thermal spreader above each chip and use thermal pastes.
Patel, U.S. Pat. No. 5,587,882, is directed at a referencing scheme which provides an improved thermal interface between a heat sink and a specific chip within a multichip module by forming a pedestal, which is aligned with the chip, on the bottom surface of the heat sink. Selective referencing is provided where a chip, mounted on a substrate has a higher power density than other chips and selective referencing ensures that the chip will have the lowest thermal resistance at the heat sink to chip interface. To selectively reference the chip, a pedestal is formed on the heat sink in the region corresponding to the location of the chip, enabling a thermal transfer medium between the chip and the heat sink to be thinner than the thicker thermal transfer medium between the other chips and the heat sink. The above requires complicated support ring and pedestal structures to be formed on the cap or thermal spreader which is difficult, or very expensive, to form with diamond or Si—SiC composites or SiC.
Notwithstanding the prior art there remains a need to reduce the silicon substrate thickness of the lower power density chips, such as memory, mounted on a common wiring substrate with a high power density chip, or chip which requires a lower total thermal resistance, such as a processor, so that when a thermal heat spreader made of a hard, brittle, low thermal expansion and high thermal conductivity material with a polished bottom surface, such as diamond, Si—SiC composite, or SiC is rigidly bonded onto the back surface of the chips mounted on the common wiring substrate, the high power density chip has the lowest thermal resistance to the thermal spreader.
With materials such as diamond, Si—SiC composite or SiC, or composites containing particles of these materials, the cost of a thermal spreader or lid where a pedestal is formed results in a very large cost increase due to the difficulty in machining such hard and brittle materials. Wafer thinning, by grinding and/or polishing, is a standard semiconductor manufacturing process and adds only minimal cost to each chip when done at the wafer level. The amount of material removed from the wafer containing the low power density chips is adjusted to ensure that the high power density chip projects farthest above the wiring substrate than the low power density chip(s). This enables the use of diamond, SiSiC, SiC, or similar thermal spreaders with planar bottom surfaces to be used with a combination of high and low power density chips on a common wiring substrate with a simple and effective packaging structure.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.